For the mechanical members that are constituted by planetary roller screws and bearings, Document FR 2 925 640 discloses having recourse to a liquid lubricant, preferably an oil with diluted additives, suitable for being distributed by capillarity between the screw, the planetary rollers, and the nut in order to lubricate those elements.
Planetary roller screws and bearings are sometimes used in applications where they are subjected in operation to large axial loads and to movements that are small and/or slow. This applies for example when a planetary roller screw or bearing actuates a moving element of an aircraft such as a flight control surface.
It is found that liquid lubricants are of little or no effect in the presence of large axial loads, in particular when associated with movements of the planetary roller screw or bearing that are small and/or slow. It is extremely difficult under such conditions to obtain a uniform film of oil between the planetary rollers and the screw or the nut of the planetary roller screw or between the rollers and the rings of the planetary roller bearing, thereby greatly degrading the operation of the planetary roller screw or bearing. This drawback is made worse when the liquid lubricant has low viscosity.
Proposals have also been made to lubricate such a planetary roller screw or bearing by means of grease. Grease withstands higher axial loads and is well adapted to movements that are smaller and/or slower. In contrast, its performance is not always maintained long enough without maintenance for certain particular applications such as applications in the field of aviation.
Furthermore, grease presents numerous other drawbacks, in particular the fact of having viscosity that varies greatly with temperature. Thus, at a temperature close to −30 degrees Celsius (° C.), most, greases tend to freeze, thereby slowing down or preventing rotation of the planetary rollers. Unfortunately, when a planetary roller screw or bearing is on board an aircraft, e.g. to actuate a flight control surface, the planetary roller screw or bearing is subjected to low temperatures of about −50° C., on average. Lubrication by means of grease is therefore inappropriate for such an application.
Furthermore, grease tends to become expelled away from the various zones of contact between the planetary rollers and the screw or the nut, or between the rollers and the rings of the bearing it is therefore necessary to refill the planetary roller screw or bearing regularly with grease in order to limit that phenomenon.
There also exist solid lubricants. Consideration has been given to having recourse to a so-called “dry” solid lubricant, that consists in covering portions of the mechanical member, such as the screw and the nut and/or the planetary rollers in a coating of solid lubricant. In a first embodiment, particles of solid lubricant are incorporated in the screw and the nut and/or the planetary rollers so that the particles become released progressively as a result of wear. By way of example, the coating comprises molybdenum bisulfide (MoS2). In a second embodiment, the lubricant coating is deposited on the surface of the screw and the nut or of the planetary rollers. By way of example, the coating comprises tungsten bisulfide (WS2).
Although solid lubricants are better than liquid lubricants at withstanding higher axial loads associated with movements that are smaller and/or slower, they present the drawback of being extremely sensitive to the climatic, thermal, and/or chemical conditions that are to be encountered in numerous industrial fields, such as the field of aviation, for example.
In particular, solid lubricants tend to degrade in the presence of water and thus lose all their effectiveness. Water can sometimes even make solid lubricants aggressive relative to the surfaces on which the solid lubricants are arranged. With certain coatings, e.g. with MoS2 or WS2 coatings, this degradation leads to two aggressive components being formed, namely an abrasive oxide and sulfuric acid.
In addition, a gaseous compound can infiltrate into the planetary roller screw or bearing and can condense inside said screw or bearing in the form of a liquid that might react with the solid lubricant. In general, chemical compounds can spoil any lubricant, but they act more aggressively in spoiling solid lubricants. Thus, the mere presence of a gaseous chemical compound may suffice to degrade the solid lubricant, in particular in the zones of contact in the planetary roller screw or bearing, where friction in said zones amplifies the reactivity of the gaseous chemical compound.
The sensitivity of solid lubricants to thermal, climatic, and/or chemical conditions can thus have severe consequences concerning the operation of the planetary roller screw or bearing so as to make it difficult to have recourse to such lubricants in certain applications such as the field of aviation.